This invention relates to an automatic transmission, particularly to a control system of an automatic transmission for controlling coupling pressure to friction coupling members.
There has been known an automatic transmission comprising a transmission gear mechanism including a planetary gear unit and friction coupling members for shifting the gear ratio, a hydraulic control circuit for developing hydraulic pressure to the respective coupling members, the hydraulic control circuit including regulator means and a duty solenoid valve so that the hydraulic pressure to the respective friction coupling members is controlled by actuating the duty cycle of the duty solenoid valve. For example, Japanese Examined Patent Publication No. 54-2349 discloses such an automatic transmission.
In such an automatic transmission, the coupling pressure to the respective coupling members is controlled by actuating the duty solenoid valve in accordance with the running condition. In controlling the coupling pressure to the friction coupling members by the use of the duty solenoid valve, there have been generally two control manners.
One control manner is a feed-back control. Specifically, with respect to hydraulic pressure in connection with the coupling pressure, a target hydraulic pressure suitable for each of various running conditions is set in advance. An actual hydraulic pressure is detected by use of a hydraulic pressure sensor. The actual hydraulic pressure is compared with the target hydraulic pressure. The solenoid valve is controlled so that the difference between the actual hydraulic pressure and the target hydraulic pressure may become zero. However, the feed-back control is unproper in a state of changing the gear speed which requires rapid response.
The other control manner is an open control. The open control is rapider in response than the feed back control. In the open control, target duty cycles of the solenoid valve which ensure respective suitable coupling pressures for various running conditions are determined and stored in advance, e.g., in the manufacturing stage. When changing the gear speed, the target duty cycle for the present running condition is retrieved and the solenoid valve is then controlled so as to have the target duty cycle. However, there is the following problem in the open control: The performance of the solenoid valve or the like is liable to vary with use over time, which then changes the relationship between the duty cycle and the coupling pressure. Consequently, the coupling pressure cannot be controlled accurately.